1. Field of the Invention
The present invention relates to a servo apparatus which generates a driving instruction for driving a motor based on an input indicated angle, and provides the generated driving instruction to the motor to drive and control the motor, and relates to a controlling method of the servo apparatus.
2. Description of the Related Art
In recent years, for example, in a robotic cell apparatus which is equipped with a plurality of multi-joint robots and works just like a human, the number of the multi-joint robots tends to increase, and thus a small and inexpensive servo apparatus is required as the servo apparatus to be used for each joint of the multi-joint robot. For this reason, in the industrial multi-joint robot, a servo apparatus which has been developed from a servo apparatus for the purpose of hobby is often used as the servo apparatus for driving the joint of the robot.
The servo apparatus (e.g., “RS405CB” manufactured by Futaba Corporation) for the robot comprises, e.g., a brushless DC (direct current) motor, a potentiometer for detecting output rotation of the DC motor, and a controlling unit. More specifically, if an indicated angle is input by a serial signal according to the RS-485 (recommended standard 485) or the like, the controlling unit of the servo apparatus performs feedback control to the motor based on the detected angle from the potentiometer, and concurrently controls the angle position of the motor by a torque controlling (elastic controlling) method. Incidentally, the potentiometer detects a voltage change of a resistor in an analog manner, and performs AD (analog-to-digital) conversion to convert the detected voltage change (analog signal) into a digital signal with, e.g., 12 bits (4096 stages), thereby achieving resolution of 0.1 degrees (3600 stages).
Incidentally, to increase resolution of a shaft angle (also called shaft angle resolution, hereinafter), it is considered to also increase resolution of the AD conversion. However, even if the resolution of the AD conversion is further increased, it is impossible to increase the shaft angle resolution because the detected value is unreliable due to noises. Under the circumstances, it is hard to further increase the shaft angle resolution as long as noise reduction in the AD conversion is not remarkably improved.
On another front, Japanese Patent Application Laid-Open No. 2002-165493 proposes a stepping motor which enables micro-step driving. In this case, the stepping motor can perform position control of a shaft angle between full steps by changing over adjacent two full-step excitation states based on time ratio.
As for the relation between the indicated angle and the torque in the servo apparatus such as RS405CB manufactured by Futaba Corporation, it is impossible to perform the torque control of the angle position which is less than the resolution. On another front, in recent years, there is a case where it is required to mount a camera to the tip of the multi-joint robot and minutely move the tip position of the multi-joint robot based on the image taken by the camera during work or the like. For example, an image which was taken by the camera having 2048×1536 pixels at a distance of 50 mm has the pixel pitch of about 28 μm. However, for example, in a case where the distance of the joint moving at the tip position of the multi-joint robot is 200 mm from the tip position, if control is performed at the angle position of 0.1 mm, then the moving resolution thereof is about 350 μm. Therefore, even if the resolution in image recognition is two to three pixel pitches, it is required to set the resolution of the angle position control of the servo apparatus to at least ¼ or less.
On another front, since the stepping motor performs driving with angles obtained by equally dividing a mechanical angle (shaft angle) by an electrical angle, there is a problem that the stepping motor is not appropriate for a high-speed rotation operation. In this connection, in a case where the stepping motor which is difficult to perform the high-speed rotation operation is intended to be used for the industrial multi-joint robot, if a speed reducer lies therein, the driving speed of the stepping motor decreases as a whole, whereby production speed decreases resultingly. In order not to use the speed reducer to prevent such inconvenience, it is necessary to increase the output torque of the stepping motor. In such a case, when the stepping motor is mounted for the joint of the multi-joint robot, it is necessary to enlarge the size of the stepping motor itself in consideration of output performance, prevention of step out, and the like. Consequently, it is difficult to adopt the stepping motor from the aspect of mountability.